Xenobiotic metabolism results principally in detoxication. However, in certain instances, bioactivated and highly toxic intermediates are generated. Cellular levels of epoxide moieties resulting from chemical metabolism appear to be critical initiators of toxic damage, including genetic mutation. As a case in point, polyaromatic hydrocarbons (PAHs) are products of incomplete combustion of organic matter, are widespread environmental contaminants and are considered procarcinogens because they require metabolic activation to electrophilic reactive metabolites to exert their mutagenic and tumorigenic activities. Microsomal epoxide hydrolase, EPHX1, bioactivates PAHs, contributing to the formation of highly reactive and carcinogenic bay region and fjord region diol-epoxide intermediates. Previously, we characterized the gene structure for human EPXH1 and identified two structural polymorphisms, an exon 3 polymorphism encoding Y/H substitutions at amino acid position 113, and an exon 4 H/R substitution at position 139. Results of many molecular epidemiological studies have since associated EPHX1 allelic variation as a risk factor for various diseases, most notably lung cancer. Recently, a paradigm-shifting report from our laboratory demonstrated that the expression of the human EPHX1 gene is driven by the use of alternative promoters, with a far upstream promoter, termed E1-b, preferentially driving expression EPHX1 in most human tissues. New findings demonstrate that the E1-b promoter is itself genetically polymorphic and is transcriptionally modulated by dietary chemopreventive agents, such as sulforaphane. In this research program, we have designed experiments to test several key and integrated hypotheses, including: 1) that the most common structural genetic variants of EPHX1 (Y113H; H139R) confer their association with PAH-induced cancer risk by nature of their substrate-specific catalytic activity among PAH epoxides, specifically towards fjord-region epoxides that exhibit extraordinarily high tumorigenic potential; 2) that the associated transcriptional activity of the E1-b promoter region is interindividually regulated by the presence of genetic polymorphism and transposable genetic elements in tissues that are principle targets of xenobiotic-induced toxicity; and, 3) that EPHX1 expression in human tissues is differentially regulated by exposures to isothiocyanate derivatives, dietary substances under current study as chemoprevention agents. The results of these investigations will delineate critical features comprising the enzymology, transcriptional regulation and genetics of human EPXH1 and elucidate the functional roles of these processes as potential risk modifiers of human disease. PUBLIC HEALTH RELEVANCE: Microsomal epoxide hydrolase is an enzyme that is present in most tissues and plays a key role in metabolizing numerous environmental chemicals. The genetics and regulation of this enzyme likely determines, in part, the nature and extent of interindividual differences in toxicity that result from chemical exposures, including cancers. This research program will characterize the critical features that account for these differences and investigate their roles as risk modifiers of human diseases.